1. Field of the Invention
This invention pertains generally to furnaces, and more particularly to furnaces that incorporate screw-type fuel feeders. In a particular manifestation of the invention, corn kernels are used as a fuel source.
2. Description of the Related Art
Thermal energy has many fundamental applications, ranging from basic necessities such as adequate warmth within a shelter to comfort and pleasantries such as hot water used in baths, spas and swimming pools. Representative of the breadth of applications are the diverse apparatus that have been devised to provide desired thermal energy. Common modern sources of thermal energy include electricity such as is typically produced at large electrical generation power plants, propane, kerosene, fuel oil, other petroleum-derived compounds, coal, natural gas, and wood. More recently, various easily renewable materials have been pursued which might provide the necessary fuel source for production of thermal energy. Among the fuel sources considered, which are too great to individually list herein, are biomass pellets which may be manufactured from diverse organic-based sources such as plant materials and crop residues. Corn, which is available naturally as kernels, has presented another opportunity for a readily renewable resource.
In spite of ready availability, and often times extremely competitive pricing per unit of energy produced, the use of corn as a fuel source has presented several challenges. One such challenge is the delivery of the corn kernels to the combustion chamber, hereinafter referred to as the fire pot. Unlike prior art liquid fuels, which may be delivered absent air or oxygen, corn kernels will, if not further processed, present small spaces and gaps between kernels which in turn entrap air. As some prior art burners have demonstrated, the temperature within a fire pot is sufficient to heat and ultimately ignite kernels within an auger feeder, owing to the availability of the air therein. One approach has been to vent air directly through the fuel source, in this case corn kernels, and into the fire pot. Exemplary of such air flow through the auger tube is illustrated in U.S. Pat. Nos. 4,619,209 to Traeger et al and 5,123,360 to Burke et al, the contents which are incorporated herein by reference. By such technique, the kernels will desirably be cooled sufficiently to prevent the back-spreading of fire into the hopper area of the furnace. Unfortunately, it is not always possible to control how tightly the kernels may be packed within the auger. Consequently, it is also not possible to reliably control the flow of air there through, nor to predict the temperature therein.
Another limitation of pellet furnaces in general, and also corn burners, is the difficulty with initial ignition and start-up of the furnace. Solid pellets or kernels are not readily mixed within an air stream, and so consequently cannot simply be sprayed and ignited with a spark or the like. Instead, the solid fuel is more commonly decomposed within a very hot fire pot, and the resultant gases combusted to produce the desired thermal energy. In order to obtain this sequence, the fire pot must be at a sufficiently elevated temperature to enable thermal decomposition. One method of obtaining this temperature is to use an electric heater, referred to commonly as an ignitor, to heat a location within the fire pot to the substantial temperature required for proper combustion. Once the localized region heats and ignites, the energy released therefrom will similarly be useful to support combustion across an even larger area within the fire pot. Eventually, it is desirable to have as large a region within the fire pot heated as possible, though using the prior art burners this has not been practical. In some prior art designs, ignitors have remained within the fire pot for the entire operation of the furnace. Unfortunately, this exposes the ignitor to continuously elevated temperatures, which tends to degrade the ignitor unnecessarily. Furthermore, the physical placement of the ignitor, which is usually selected to be in as close a proximity to the solid fuel as is reasonably practical, will interference once the combustion process has actually begun and attained a self-supporting status. Commensurate therewith, there have been a few designs in the prior art that have provided for the removal of these ignitors once combustion has become self-sustaining with the fuel pellets. Nevertheless, the control of these ignitors has heretofore required expensive equipment which has been of little use or benefit other than for the few seconds of use inserting or removing the ignitor. Owing to the time lag typical with the proper ignition of these types of furnaces, they may be ignited only once or a few times during an entire heating season. Consequently, the additional hardware and mechanics that add cost are most undesired. Exemplary prior art ignitors are illustrated by U.S. Pat. Nos. 5,000,100 to Mendive et al and 5,263,642 to Orchard, the contents which are incorporated herein by reference in entirety.
Another challenge of corn burners is the requirement for proper temperature, mixing and oxygen exposure. If a mass of corn is left relatively undisturbed during the burning process, there is a great likelihood that a clinker will form. Clinkers are large, very hard clumps of spent fuel. Unfortunately, owing to the hardness and solid mass formed, a clinker will not typically further burn, and it will instead interfere with the combustion of other kernels. Finally, the presence of these clinkers represents a waste product which is undesirable, and will require further disposal. No effective solution has been provided heretofore, though U.S. Pat. No. 4,947,769 to Whitfield, the contents which are incorporated herein by reference, illustrates a rotating member to remove ash and clinkers from the combustion grate.
Yet another challenge of the prior art pellet and corn burners is that of maintaining optimum temperature control. In liquid-fueled furnaces, the furnace will generally be sized to have excess heat capacity, where the time on and off is used to determine the actual heat output. Since the flame is formed through the simple generation of a spark, starting and stopping the heating cycle is very simple. The building or space being heated is used as a thermal mass which evens out the temperature between operating cycles of the furnace. While this has in the past been associated with draftiness and lack of comfort, the approach is nevertheless made possible by the easy ignition of the fuel source. In contrast, and as aforementioned with respect to the ignitors, the starting cycle for a corn fueled furnace may be measured by many minutes or hours. Furthermore, the start-up of a corn burner is less precise and may require user intervention. Both the time and intervention required will interfere with or prevent the cycling found in liquid or gas furnaces. Instead, the furnace will preferably stay lit and will use other technique for controlling heat output. In the past, this control has either been absent, meaning the furnace has been simply run at full capacity non-stop, or there has been only limited control provided. In practice, a user has been required to select a proposed heat output for the day, based upon anticipated heating needs. For a closed building of large thermal mass, this technique can provide the necessary level of control. However, when a larger door, such as an overhead door commonplace in factory loading docks and where large machinery is stored and removed for use, there may be substantial heat loss in short time periods. The present thermal regulation of corn burners is inadequate to compensate for these short period loads. Some furnaces which attempt to include speed control are illustrated by U.S. Pat. Nos. 5,873,356 to Vossler et al and 4,856,438 to Peugh, the contents of each which are incorporated herein by reference for their teachings with regard to control systems.